[Feature] Support PSAMask with cambricon MLU backend

[zcyKTH]

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Motivation

The motivation of the PR is to support running PSAMask with Cambricon MLU backend.

It includes three parts:

Add psamask_mlu_kernel.mlu src code as mlu kernel.
Add psamask_mlu.cpp to support launching kernel in pytorch.
Refactor test_psa_mask.py to support testing psamask with MLU backend.

Modification

MLU src code
Add MLU src code of psamask in directory mmcv/ops/csrc/common/mlu/psamask_mlu_kernel.mlu.
Pytorch adaptation
Adapt psamask for pytorch in mmcv/ops/csrc/pytorch/mlu/psamask_mlu.cpp.
Unit test
Surpport test psamask with various backends in tests/test_ops/test_psa_mask.py.

BC-breaking (Optional)

Does the modification introduce changes that break the backward-compatibility of the downstream repositories?
If so, please describe how it breaks the compatibility and how the downstream projects should modify their code to keep compatibility with this PR.

Use cases (Optional)

If this PR introduces a new feature, it is better to list some use cases here, and update the documentation.

Checklist

Before PR:

• I have read and followed the workflow indicated in the CONTRIBUTING.md to create this PR.
• Pre-commit or linting tools indicated in CONTRIBUTING.md are used to fix the potential lint issues.
• Bug fixes are covered by unit tests, the case that causes the bug should be added in the unit tests.
• New functionalities are covered by complete unit tests. If not, please add more unit test to ensure the correctness.
• The documentation has been modified accordingly, including docstring or example tutorials.

After PR:

• If the modification has potential influence on downstream or other related projects, this PR should be tested with some of those projects, like MMDet or MMCls.
• CLA has been signed and all committers have signed the CLA in this PR.

[grimoire]

Compile failed with log:

./mmcv/ops/csrc/common/mlu/psamask_mlu_kernel.mlu:117:28: error: no matching function for call to 'max'
        const int hstart = std::max(0, half_h_mask - h_abs);
                           ^~~~~~~~

cncc version :

cncc 3.6.4 clang version 11.0.0

[zcyKTH]

Compile failed with log:

./mmcv/ops/csrc/common/mlu/psamask_mlu_kernel.mlu:117:28: error: no matching function for call to 'max'
        const int hstart = std::max(0, half_h_mask - h_abs);
                           ^~~~~~~~

cncc version :

cncc 3.6.4 clang version 11.0.0

I have replaced std::min/max with conditional operations, which is similar to #2033.